It has already been proposed in the prior art to provide apparatus which enables the power dissipated by a discharge lamp to be continuously measured. The reliability of a lighting circuit, as well as the light output of spotlights or headlights, depends largely on the power that is supplied to the discharge lamp from the power supply means. In practice, it is necessary that the power that is supplied to the discharge lamp must be strictly controlled during discharge.
In the prior art, it has also been proposed to provide apparatus which enables both the voltage and current passing through a resistive load to be measured simultaneously. However, in the kind of context to which this invention relates, the hot point of the discharge lamp is at a high voltage. For this reason, direct measurement is not considered to be practicable for motor vehicles.
A further type of arrangement has also been proposed, in which the current passing through the discharge lamp, and the voltage across the terminals of a group of a few selected turns at the foot of the secondary of the supply transformer for the discharge lamp, are measured. However, such a solution has a number of drawbacks which will be discussed below. In particular, such a solution calls for the use of a four-quadrant analogue multiplier. Thus the voltage and current which are measured are not in phase, so that the algebraic product of the measured current and measured voltage has to be calculated.
Multiplication circuits of the kind mentioned above require both positive and negative supplies, thus making the use of this kind of solution in motor vehicles somewhat complex.